1. Field of Invention
The present invention relates to a technical field of environmental protection, especially belongs to a technical field of soil contamination treatment, and more particularly to a dithiocarbamate functionalized dendrimer with an alkylenediamine core as a soil heavy metal immobilization amendment and a preparation method thereof.
2. Description of Related Arts
Along with the continuous expansion of the industrial production scale and the rapid development of the urbanization in China, the soil is suffering from various degrees of heavy metal contaminations. The soil heavy metal contamination mainly comes from the human activities, such as mining, smelting, electroplating, tanning and metal processing.
The Report on the national general survey of soil contamination, firstly published on Apr. 17, 2014, by the Ministry of Environmental Protection and the Ministry of Land and Resources of the People's Republic of China, indicates that 16.1% of the surveyed land of 6.3 million square kilometers (about 65.6% of the territorial area, covering all the arable land and part of the forest land, the grassland and the construction land of China) is contaminated, wherein 82.8% of the contaminated land is contaminated by the heavy metals; and 19.4% of the arable land is contaminated, mainly by the pollutants of cadmium (Cd), nickel, copper, arsenic, mercury and plumbum (Pb), which means that nearly one fifth of the arable land suffers from the heavy metal contamination. In China, the soil heavy metal contamination leads to a decrease of 10 million tons in the food production, and 12 million tons of food is contaminated by the heavy metals.
Unlike the organic pollutants, the non-degradable heavy metals are unable to be remedied through a microbial degradation or a chemical degradation. Thus, the soil heavy metal contamination is accumulative and relatively stable, which leads to a relatively difficult remediation of the soil contaminated by the heavy metals.
Generally, the soil heavy metal remediation is divided into the physical remediation, the bio-remediation and the chemical remediation.
The physical remediation mainly comprises: agricultural engineering measures (soil replacement and soil dressing), heat treatment, vitrification and electrokinetic remediation. The agricultural engineering measures are suitable for the small area of the contaminated soil. However, because of involving digging and moving soil, the agricultural engineering measures consume massive manpower, material resources and financial resources; and the replaced soil also needs to be treated. Thus, the agricultural engineering measures have a high treatment cost; it is difficult to treat a deep pollution by the agricultural engineering measures, especially for a situation that the pollutants permeate into the saturated layer soil and the underground water; and the agricultural engineering measures destroy the original soil structure and the ecological environment. The heat treatment has a high energy consumption and a high operation cost, and is merely suitable for volatile pollutants. The vitrification is expensive. The vitrified soil still needs to be landfilled, which occupies lands and wastes resources. The electrokinetic remediation is merely suitable for the sandy soil and expensive.
The bio-remediation, through the metabolism of the organisms (microorganisms, plants and animals), absorbs and removes the heavy metals in the soil or transforms the heavy metals, so as to reduce the toxicity and purify the soil. The microorganism remediation is an emerging soil remediation method, but still in the research phase because of a high cost and a difficult microbial strain selection. The lower animals for remediating the soil are limited in the number, and the lower animals after absorbing the heavy metals may be released into the soil again to cause a secondary pollution. The phytoremediation accumulates the heavy metals in the soil by accumulators and hyperaccumulators, and the plants are collected and centralized to detach the heavy metals from the contaminated soil, so as to remove the heavy metals in the contaminated soil. The phytoremediation is the most promising original ecological soil treatment technology at present, but is obviously disadvantageous as follows. Firstly, the hyperaccumulators grow slowly and have a long remediation period, generally 3-5 years; secondly, the phytoremediation is merely able to accumulate one specific heavy metal or some specific heavy metals, but the soil is generally contaminated by a combination of the heavy metals; thirdly, the soil rich in the heavy metals impairs the growth of the hyperaccumulators, leading to the low heights, the low biomass, the slow growth and the long growth period of the hyperaccumulators, so as to decrease the remediation efficiency and the remediation rate; and fourthly, the hyperaccumulators need to be harvested and properly disposed as waste, otherwise may threaten the biodiversity to a certain degree.
The chemical remediation comprises chemical washing remediation and in-situ chemical immobilization remediation.
The chemical washing remediation washes the soil with washing agents (surfactants, organic acids and salts thereof, and chelating agents) to transfer the heavy metals from the soil to the waste water, and then the waste water is recycled and treated, so as to remedy the soil. However, the chemical washing remediation is expensive; while removing the heavy metals, the chemical washing remediation leads to a nutrient loss of the soil, and carries the washing agents into the soil, becoming another contamination resource. The chemical washing remediation destroys the soil structure, contaminates the underground water, and leads to a high environmental risk.
The in-situ chemical immobilization remediation is to add chemical reagents or chemical materials into the soil. Through absorption, precipitation, complexing, ion exchanging and a redox reaction, the heavy metals are turned into insoluble substances having a low solubility and a low dissolution rate, which reduces the mobility of the heavy metals in the soil and the possibility to be absorbed by the plants. Thus, usually the chemical materials are also called amendments. The heavy metals are immobilized stably in the soil through the amendments, so as to realize treating and remediating the contaminated soil.
Compared with other remediation measures, the in-situ chemical immobilization remediation of the soil with the amendments has advantages of the wide application scope, the high processing capacity, the fast yield, being economical and convenient, and maintaining the inherent physical and chemical properties of the soil unchanged. The in-situ chemical immobilization remediation is considered as a potential alternative technology which is able to reduce the health risk of the heavy metal environment, and thus has a good application prospect.
The conventional amendments are mainly divided into the inorganic amendments and the organic amendments. The inorganic amendments mainly comprise minerals (sepiolite, bentonite, montmorillonoid, zeolite powder, attapulgite, diatomite and metallic oxide ores such as aluminum oxide ore, manganese oxide ore and iron oxide ore), phosphates (phosphate, hydrophosphate and sodium dihydrogen phosphate), alkaline substances (calcium oxide, calcium hydroxide, carbonate and bicarbonate), sulfides (polysulfide, hydrosulfide salt and sulfide) and biochars (straw charcoal powder, rice husk charcoal powder and bamboo shell charcoal powder). The organic amendments comprise humic acids, organic fertilizers and chelating agents.
The published amendment is usually one of the conventional amendments or a combination of at least two of the conventional amendments. The Chinese patent application, CN 200910070713.4, discloses a combined amendment comprising sepiolite and dicalcium phosphate, which is able to immobilize the soil having a total Cd content of 1.2 mg/Kg and an available Cd content of 0.6 mg/Kg. When the addition dosage of the combined amendment reaches 9% of the soil, the available Cd content is decreased to 0.23 mg/Kg by 61%. The Chinese patent application, CN 201210227669.5, discloses a combined amendment comprising slaked lime, monopotassium phosphate, plant ash and silkworm excrement, for immobilizing and remedying the soil containing Pb and Cd. When the addition dosage of the combined amendment reaches 2% of the soil, the available Pb content is decreased by 47.61% and the available Cd content is decreased by 50.57% after 30 days. The Chinese patent application, CN 201110203080.7, discloses a combined amendment comprising calcium hydroxide, fly ash, sodium carbonate, polycrylamide and sodium sulfate, for processing the soil with an in-situ immobilization remediation. When the addition dosage of the combined amendment reaches 5% of the soil, the leaching content of Pb, Zinc (Zn), Cd and Copper (Cu) is respectively decreased from 2.86 mg/L, 1.37 mg/L, 0.31 mg/L and 4.65 mg/L to 0.35 mg/L, 0.42 mg/L, 0.03 mg/L, 0.42 mg/L and 0.84 mg/L. The Chinese patent application discloses a combined amendment comprising humic acid mineral powder, calcium magnesium phosphate fertilizer and quicklime. When the addition dosage of the combined amendment reaches 3% of the soil, the available Cd content in the soil is decreased from 8.90 mg/Kg to 7.38 mg/Kg. The Chinese patent application, CN 201210244674.7, discloses a combined amendment comprising rice husk charcoal, powdered straw and sifted fly ash. When the addition dosage of the combined amendment reaches 8% of the soil, the available Pb content in the soil is decreased by 73.25%.
The above conventional amendments have the following limitations. Firstly, for the heavily contaminated soil, the immobilization effect of the conventional amendments is negligible. Secondly, the conventional amendments are unable to guarantee a long-term and stable existence of the remedied soil in the nature. When the environment changes, the immobilized heavy metals may be activated and then be dissolved out again, causing a secondary contamination.
Thus, it is urgent to provide a highly-efficient amendment with an enhanced ability to stabilize the heavy metals with regard to the research of the in-situ remediation amendment of the heavy metals in the soil.
With the development of the polymers, based on the conventional one-dimensional linear, two-dimensional crosslinked or slightly branched polymers, the highly-branched polymers having a three-dimensional spatial structure are developed. According to the structure, the highly-branched polymers are divided into the dendrimers and the hyperbranched polymers. The dendrimers have a regular structure, but the hyperbranched polymers have an irregular structure. In the 21st century, the highly-branched polymer has attracted more and more attentions of the scientists around the world, because of the special structure. The highly-branched polymer has an important application prospect in the fields of industry, agriculture, national defense, biomedical, sustained-release materials and catalysis.
The present invention adopts a dithiocarbamate functionalized dendrimer with an alkylenediamine core as a soil heavy metal immobilization amendment, which has good performance in stabilizing the heavy metals in the soil, has an ability to reduce the available heavy metals and the carbonate bound forms of the heavy metals and to well resist acid rain, and guarantees a long-term stability of the remedied soil in the environment.